In a process of manufacturing a highly integrated device such as a semiconductor device or liquid crystal display device, a reduction projection exposure apparatus using a reduction projection optical system to transfer a circuit pattern drawn on an original onto a substrate coated with a photosensitive agent is adopted. To further increase the degree of integration of a semiconductor, an exposure apparatus is demanded to resolve a further micronized pattern. In order to attain high resolution of the exposure apparatus, it is common practice to shorten the exposure wavelength or increase the numerical aperture (NA) of a projection optical system. As the NA increases, the depth of focus DOF decreases. The relationship between these factors can be expressed by:resolution R=k1λ/NAdepth of focus DOF=±k2λ/NA2 where λ is the wavelength of exposure light, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients.
As for the exposure wavelength, an i-line having a wavelength of 365 nm, a KrF excimer laser which emits light having a wavelength of 248 nm, an ArF excimer laser which emits light having a wavelength of 193 nm, and an F2 excimer laser which emits light having a wavelength of 157 nm have been developed, so an increase in resolution of the exposure apparatus has been realized. To achieve a higher resolution in the future, an exposure apparatus using a soft X-ray (also called EUV (Extreme Ultra Violet) Light) having a wavelength of 13.5 nm is also under development.
As for the numerical aperture (NA) of the projection optical system, as disclosed in Japanese Patent Laid-Open No. 06-124873 and WO 99/49504, a liquid immersion method has received a great deal of attention, which fills the region between the exposure target substrate and the exit surface of the projection optical system with a liquid. As is obvious from the above relation and a nature in which the NA is proportional to the refractive index of the region, the resolution of the exposure apparatus is inversely proportional to the refractive index of the region. The liquid immersion method is implemented using this nature. Letting λ0, λa, and λ1 be the wavelengths and n0, na, and n1 be the refractive indices of exposure light in a vacuum, air, and an immersion liquid, respectively, the relationship between the resolution R1 and the depth of focus DOF1 in liquid immersion exposure is expressed by:resolution R1=k1λ1/NA=k1(λ0/n1)NA=k1(λa/na)/n1NA≈k1λa/n1NA (from na≈1)depth of focus DOF1=±k2λ1/NA2=±k2(λ0/n1)/NA2=±k2(λa/na)/n1NA2≈±k2λa/naNA2 (from na≈1)=±n1 k2λa/(n1NA)2 
As is obvious from the above equation, the use of the liquid immersion method increases the resolving power to 1/n1 times. This amounts to using exposure light having a wavelength 1/n1 times or using a projection optical system having a numerical aperture n1 times without using the liquid immersion method. As for the depth of focus, the use of the liquid immersion method increases it to n1 times that without using the liquid immersion method.
In the above liquid immersion method, an imaging failure may occur when micro-bubbles enter the exposure optical path in an immersion liquid and shield it. Also, since the micro-bubbles burst at a high pressure upon extinction, a photosensitive film may be damaged by the impact of the pressure. By excessively degassing the liquid in a vacuum using a degasser, it is possible to shorten the life of the generated micro-bubbles. However, since the degasser has a structure which requires a space to accommodate, it must be installed at a position largely spaced apart from the substrate. Therefore, micro-bubbles may be generated in the path from the degasser to the substrate when turbulence is generated due to the unevenness of a pipe or nozzle or when a gas is trapped in the liquid in a region where the gas comes into contact with the liquid. As the amount of micro-bubbles generated gets higher than the saturated dissolution amount in the liquid, the generated micro-bubbles remain in the liquid. Even if all the micro-bubbles generated can be dissolved in the liquid, a predetermined time is required for dissolution to occur. Therefore, an imaging failure may occur upon shielding the optical path before dissolution. Also, a photosensitive film may be damaged by the impact of a pressure acting upon extinction of the bubbles.